Stabilization of rubbers



Patented Jan. 8, 1952 HSTABILIZATION OF RUBBERS George E. P. Smith, Jr. and Harry E. Albert, Akron, Ohio, assignors to The Firestone Tire & Rubber Company, Akron, Ohio, a corporation of Ohio No Drawing.

Application January 10, 1948. Serial No. 1,673

19 Claims. (C1. 26045.75)

invention relates to the stabilization of rubbera b'oth natural rubber and rubbery copoly- =i1e'rsbf a vinyl monomer and a conjugated-diene monomer. The rubbery copolymer of 1,3-butadiene'and styrene known as GR-S is representative of such a copolymer. The rubbery copolymer o'f 1,3-butadi'ene and acrylonitrile also belongsto the general class. The copolymers which may be stabilized according to this invention include the rubbery copolymers of a conjugated-diene monomer-for example, l,3-butadiene; isopre'ne, 2-cyanobutadiene-1,3, cyclopentadiene, piperylene, 'dimethyl butadiene-l,3, 2- 'methyl-l, 3-pentadiene, etc. and a vinyl mono mer-for example, aromatic monomers such as styreney'alpha' methyl styrene, nuclear-substituted "styrenes, monochlorostyrene, dichlorostyren, vinylnaphthalene, vinylbiphenyl, vinylcarbazole, 2-vinyl-5-ethylpyridine, 2-ethyl-5-vinylpyridine, etc, and aliphatic monomers such as monomers having the formula CH2:C(R) --X when-R is hydrogen, methyl, ethyl, propyl or chlorine, and X is COOR', COR' or CONH when R is an alkyl group of one to five carbon atom-s.- More particularly, the invention relates to the stabilization of I natural rubber and the rubbery copolymer of LS-butadiene and styrene known as GEE-S. The invention includes the process of stabilization as well as the stabilized products.

The stabilizers employed are stannous salts of alicyclic carboxylic acids, as, for example, stannous salts of various naphthenic acids,.stannous rosinate, stannous hydrorosinate, stannous dehydrogenated rosinate, and stannous salts of cyclohexane carboxylic acid. dicyclohexane carboxylic acids,- decahydronaphthalene carboxylic acids, dicyclobutane carboxvlic acids, cyclohexene carboxy'lic' acids,'and cyclopentane carboxylic acids.

Naphthenic acids are obtained from petroleum by washing with alkali. They consist principally of monoand di-cyclic reduced rings (cyclopentane mostly, but some cyclohexane and cyclobutane) with numerous aliphatic side chains and with acid groups attached either to the ring or toa-sidechain. Only a very few of these acids have been positively identified. However, the acid mixture and its neutralization equivalent are commonly known and used and can be idengified-iromthe above information. There are t ie'ast" two naphthenicacid's on the market: one advertised as having a neutralization equivalent range of 210-220 and molecular weight of 255-267, and the other advertised as having a neutralization equivalent range of 230-245 and molecular weight of 229 to 244.

The stabilizers commonly recommended for the stabilization of GR-S, such as phenyl-betanaphthylamine, etc. discolor badly on aging. The stannous salts of this invention give good stabilization of GR-S and other synthetics without appreciable discoloration. Ordinarily, the stabilizer of this invention when used with a synthetic will be added to the copolymer latex befor coagulation, but it can be compounded with the copolymer by milling or in any other manner. When used with natural rubber. it will be employed in any usual manner. By adding it to the latex of a synthetic, the copolymer is stabilized during the drying of the coagulum as well as in subsequent treatments and storage. The amount of the stabilizer employed will vary, 2 percent being a common amount, although from 0.1 to 10 percent more or less, may be used.

The coagulum for the tests on synthetics was obtained by coagulation of copolymer with aluminum sulfate, coagulant low in iron being used for the light-colored stocks. The following table refers to the results obtained on mixing 2 per cent of different stabilizers with a latex obtained by emulsion copolymerization of 1,3-butadiene and styrene, coagulating, drying and heat-aging the coagulum. The stabilizers tested included phenyl-beta-naphthylamine which was used as a control. Experience has shown that copolymer similarly compounded, except that it contained no stabilizer, would harden and resinify if dried or hardened in the manner the test materials were treated, as described in Tables I and II. In thesetables the first column of each of the recorded results gives the plasticity as determined in an extrusion plastometer of the typedescribed in J. Dillon et al. U. S. Patent 2,045,548 known generally as a. Firestone Extrusion-Plastometer; the figure given being the number of minutes required to extrude a constant volume of rubber through a given opening by the action of a piston activated by a constant pressure of 4% pounds steam at a constant temperature of F. Following the plasticity figure, the color of the test material is recorded, and then the results of a hand test are recorded. This hand test consisted of pulling and feeling the copoly: mer sample to detect any stiffening, softening or other signs of deterioration.

TABLE I copolu r Wino Alter Drying 20 Hour: at 75 C. HeatAglng at 90 C. 7

Two Days Four Days Stabilizer T4 Color Band Test T-iK Color Hand Test T454 Color Band Test 2% stnnnous rosinatem- 4.0 Light yellow. No deterioration. 10.6 Light yellow. Nochange 29. 8 Light yellow- No change. 2% stannous naphtha- 5. White (In 10.8 Light cream do 31.4 Light cream Do.

I. 2'7; ghenylbeta-naph- 7.0 Brown do 8.2 Brown do 42.4 Brown Do.

thylamine.

The above data show that the stannous rosinate and stannous naphthenate stabilize the copolymer as well as phenyl-beta-naphthylamine under the test conditions. The color advantage for the stannous salts is quite obvious.

The accelerator used was N-cyclohexylbenzothiazyl sulfenamide. .f

In the tables of physical properties herein, the modulus and tensile strength are given in pounds per square inch.

TABLE III Copolumer aging 200%Modulus Tensile Elongation mama: Par Par Par Normal Aged Cent of Normal Aged Cent of Normal Aged Cent of Normal Normal Normal stannoushydro-rosinate 275 1,010 307 2,600 2,730 105 780 425 phenyl-beta-naphthylamine..-- 295 1,190 403 2,740 2,640 96 735 385 -52 The next table records the results of using a difierent stabilizer in the same copolymer against the same control under the same conditions'with The aging was carried out for four days in an oven at 212 F. Compared to the control, the stannous hydrorosinate has the following advancomparable results. tages:

TABLE I! Copolymer aging Alter Drying 20 Hours at 76 C. Heat-Aging at C.

Two Days Four Days Stabilizer T-fli Color Hand Test Color Hand Test T-iM Color Band Test 2% stannous hydrorosinate. 5. 6 White. No deterioration- Cream No change- 10. 2 V gry lig h t No change.

rown. 2% phenyl-betanaphthyl- 6. 6 Brow do Brown do 5. 2 Brown Do.

The new stabilizer is equivalent to the control in stabilization and discolors less.

Copolymer of styrene and 1,3-butadiene to which 2 parts per of a stabilizer of this invention was added while in latex form, was coagulated with crude aluminum sulfate, dried, and then compounded according to the following formula. A blank containing the same amount of phenyl-beta-naphthylamine was similarly compounded. The compounds were cured at 40, 60, 80 and minutes at 280 F. Aged samples were obtained by heating four days in an oven at 100 C.. The average of physical tests conducted on samples so cured are recorded in Table 111. The material was compounded according to the following formula:

FORMULA I Copolymer containing 2 parts stabilizer-.. 100.00

Coal-tar-oil softener 3.00 Pine tar 3.00 Stearic acid 3.00 Zinc oxide 5.00 Accelerator 1.20 Channel black 50.00 Sulfur 2.00

FORMULA 2 Copolymer containing 2 parts stabilizer 100.00

Stearic a 1.00 Paraflln wa 0.50 Finely divided calcium carbonate 50.00 Zinc oxide 5.00 Titanium dioxide 8.00 Sunproof w x 1.80 Green dy 2.00 Coumarone resin 2.50 Accelerators 1.40 Sulfur 1.50

The accelerators used were 1.3 parts N-cyclohexylbenzothiazyl sulfenamide and 0.1 part tetrain white-sidewall-tire stock.

Natural wa s comp ounded according to Pi? E e- 1 8, 9m

Partsby weight 100.00

Pale crepe Zinc oxide T an u d i e. Steari :l""a .-I.-..

Ultramarinegblue Sunproof' wax 2.00 Insoluble sulfur 3.00 Dlbutyl ammonium oleate 0.50 Benzothiazyl disulflde 0.40

This was used as the control. To a portion of it was added 1 per cent (based on the rubber) of stannous naphthenate. The stocks were cured 20, 40, 60 and 80. minutes at 280 F. The stocks were aged (1) 3 hours in an air bomb at 260 F.; (2) in an o'xygen bomb for 96 hours at 158 F.; and (3) 2 days in an oven at 212 F. In the following table the properties of the samples before andaIter 'these-three types of aging are listed, and thevalue; .given are averages for the values What we claim is; 1. A rubber material at the class consisting of natural rubber and rubbery copolymers of conjugated-diene monomer and vinyl monomer. which contains a stabilizing amount of the stannous salt of an alicyclic carboxylic acid.

2. Rubbery copolymer of a conjugated-diene monomer and a vinyl aromatic monomer stabl 'lized with a'small amount of the stannous salt of an alicyclic carboxylic acid.

3. Latex'oi rubbery copolymer of a conjugateddiene monomer and a vinyl monomer stabilized with a small amount of a stanonus salt of an allcyclic carboxylic acid.'

4. Coagulum fromlatex of rubbery copolymer of a conjugated-diene monomer and a vinyl aroniatic monomer stabilized with a small amount of stannous salts of a mixture of naphthenic acids. I

5. Coagulumirom latex of rubbery copoiymer oi conjugated-diene monomer and avinyl aroniatid monomer stabilized with a small amount oi the stannous salt of a rosin acid.

6. Cured, rubbery copolymer of conjugateddiene monomer and a vinyl monomer stabilized with a small amount of the stannous salt of an alicyclic carboxylic acid.

-7. Cured. rubbery copolymer of conjugateddiene monomer and a vinyl monomer stabilized with a small amount of stannous salt of a naphthenic acid.

8. Cured, rubbery copolymer of conjugateddiene monomer and a vinyl aromatic monomer stabilized with a small amount oi the stannous salt of a rosin acid.

9. Cured. rubbery copolymer of 1,3-butadiene and styrene stabilized with a small amount of stannous salts of a mixture of naphthenic acids.

10. Cured, rubbery copolymer of 1,3-butadiene obtained on the stocks subjected to different 40 and styrene stabilized with a small amount of c1 i r es.: l the stannous salt of a rosin acid.

" Natural rubber-aging c. I I Aged5amples Normal Air Bomb Oxygen Bomb Oven Control Test Control Test Control- Test Control Test Modulus:

cnsl c" strength,. 44,050 3,015 1,000 .-1,o7.5 1,175 2,125 1,800 1,050 Elongation. 600 020 5l5 535 460 430 510 520 In flexing tests the test material gave an overall improvement over the control of some 30 to 40 per cent. Stannous naphthenate is a flexfatigue resistor. 'In-a weatherometer test, the test material was somewhat superior to the control.

Thus, the stabilizers are stabilizers in both natural rubber and rubbery copolymers.

Although commercially stannous salts of a mixture of naphthenic acids, or the stannous salt of one or moreof the rosin acids (stannous rosinate, stannous hydrorosinate and stannous dehydrogenated rosinate) will ordinarily be employed, it

11. The method of obtaining latex coagulum which comprises adding to latex of rubbery copolymer of conjugated-diene monomer and a vinyl monomer, a stabilizing amount of'the stannous 'salt of an alicyclic carboxylic acidfcoagulat ing. and drying;

12. The method of obtaining uncured copolymer of 1,3-butadiene and styrene which -com+ prises adding to latex of such rubbery copolymer a stabilizing amount of stannous salts of amixture of naphthenic acids, coagulating with aluminum sulfate, and then drying.

1 13. The method of dryingthe coagulum' of .110- beunderstood that the invention is not rubbery copoiymer of 1,3-butadieneand-styrene ismserm fiiJ QQ e. u e o n or n qreof thestannoussaIts of the alicyclic carbo'xylic acids, including the terpene carboxylic acids. The :examplesare only'illustrative, the

which comprises heating the same in -;admixture with a stabilizing amount of the stannous salt of -an alicyclic carboxylic acid. 1 .---j' 14. The method of producing cured, rubbery i'riventionfbeing defined by the appemled claims. 3 copoiymer oi conjugated-dime; monomer and 'aaeipu vinyl monomer which comprises curing the co- 7 polymer with sulfur in the presence or a small ainount or a stabilizer which is the stannous salt or an alicyclic carboxylic acid.

15. The method of curing rubbery copolymer of l,3-butadlene and styrene which comprises vulcanlzing the copolymer with sulfur in the presence or a small amount of stannous salts of a mixture of naphthenic acids as a stabilizer.

16. Cured natural rubber stabilized with a small amount of the stannous salt of an alicyclic carboxylic acid.

'17. Cured natural rubber stabilized with a small amount of stannous naphthenate.

18. The method of curing natural rubber which comprises vulcanizing the same with sulfur in the presence of a small amount of stannous salt of alicyclic carboxylic acid as a stabilizer.

19. A rubbery material or the class consisting of natural rubber and rubbery copolymers of conJugated-dienemonomer and vinyl monomer.

I. GEORGE E..P. SMITH. m. mars.

REFERENCES CITED The following references are of record, in the file of this patent:

Pa'rns I STATES Number- Name 4 f Date 2,182,528 Stamatofl Dec. 5, 1939 2,307,092 Yngve Jan. 5, 1943 2,476,833 Albert July 19,.1949 FOREIGN PATENTS Number Country Date Great Britain July 29. 194s 

1. A RUBBER MATERIAL OF THE CLASS CONSISTING OF NATURAL RUBBER AND RUBBERY COPOLYMERS OF CONJUGATED-DIENE MONOMER AND VINYL MONOMER, WHICH CONTAINS A STABILIZING AMOUNT OF THE STANNOUS SALT OF AN ALICYCLIC CARBOXYLIC ACID. 